1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus, and particularly to a configuration for separating sheets having a high adhesiveness between sheets by air.
2. Related Background Art
Conventional image forming apparatuses such as copiers and printers have a sheet feeding apparatus delivering sheets stacked on sheet stacking means from the top one after another by a pickup roller as sheet feeding means, and separating the sheets one by one by a separation portion and feeding the same to an image forming portion.
If sheets are continuously fed in such a sheet feeding apparatus, cut sheets are used, but such cut sheets are normally limited to fine papers and plain papers specified by copier makers. For reliably separating such sheets one by one and feeding the same, various separation systems have been employed, and such separation systems include, for example, a separation pad system in which a feed roller is brought into contact with a friction member with a predetermined pressure to prevent double feeding.
Other separation systems include a retard separation system having a feed roller rotating in a sheet conveying direction, and a separation roller driven in a direction opposite to the sheet conveying direction with a predetermined torque and brought into contact with the feed roller with a predetermined pressure. In this system, only the top sheet of a sheet stack delivered by the pickup roller is allowed to pass, and other sheets delivered along with the top sheet are fed back to the sheet stacking means side, whereby double feeding is prevented.
For reliably separating and feeding sheets by these separation systems, for example, in the case of the retard separation system, sheets can reliably be separated one by one by optimizing a back torque and an applied pressure of the separation roller with consideration given to the frictional force of sheets to be fed.
In recent years, with diversification of sheets (recording media), demands for formation of images not only on OHP sheets, art films and the like but also on sheets such as coated sheets with the surface of the sheet subjected to a coating treatment for giving a whiteness and a gloss in response to market needs for colorization have increased.
However, when a very thick sheet is to be fed, it may be impossible to pick up the very thick sheet with its self weight posing a resistance to conveyance, resulting in a jam. For sheets composed of resin materials which are easily charged, like OHP sheets and art films, sheets mutually rub to gradually charge the surfaces of the sheets, and the sheets mutually adhere with a coulomb force during a feed operation under a low-humidity environment. Therefore, for these sheets, it may be impossible to pick up the sheet, or double feeding may occur.
For sheets with the surface of the sheet coated with a coating material consisting of paints and the like, sheets are mutually attracted in nature particularly when stacked in an environment under high humidity, and therefore it may be impossible to pick up the sheet, or double feeding may frequently occur.
In the case of these special sheets, the frictional force between sheets is in itself equivalent to or less than the frictional force for plain papers. However, due to an attracting force by frictional charging under a low-humidity environment in the case of resin material sheets, and an attracting force under a high-humidity environment in the case of coated sheets, sheets are attracted with a force much greater than the frictional force between sheets, and therefore cannot be fully separated by the conventional separation system. Namely, in the case of the conventional separation system, only the frictional force between sheets is considered, and therefore sheets cannot be reliably separated if such an attracting force other than the frictional force acts.
Thus, for releasing such a very high attracting force between sheets, there is a technique disclosed in, for example, Japanese Patent Application Laid-Open No. 2004-142881. In this technique, sheets are loosened in advance by blowing air to the side face of a sheet stack to eliminate attraction between the sheets, the sheets are then picked up one by one in descending order of the position of the sheet, and the sheets are separated one by one by a separation portion provided in the downstream. Apparatuses employing such a separation and feeding system are employed in the printing industry and some of copiers. In the separation and feeding system having such means for blowing air to the side face of the sheet stack (hereinafter referred to as auxiliary air loosening means), even sheets (recording media) having a high attracting force as described above can be loosened to eliminate the attraction before feeding the sheets. Therefore, the separation performance is improved compared to the aforementioned system using only a frictional force.
FIG. 15 shows the configuration of a sheet feeding apparatus comprising such auxiliary air loosening means. Feeding a sheet S on a sheet stacking mount 60, the sheet feeding apparatus first lifts the sheet stacking mount 60 until the sheet S1 at the top on the sheet stacking mount is detected by a sensor (not shown), and temporarily stops the sheet stacking mount 60 when the sensor detects the topmost sheet S1.
After the sheet stacking mount 60 is thus stopped, air is blown in the direction shown by the arrow from auxiliary air loosening means (not shown), whereby the front end of the top sheet S1 on the sheet stacking mount is raised. After the front end of the top sheet S1 is thus raised to separate the sheet, the sheet feeding apparatus lifts the sheet stacking mount 60 until the front end of the sheet is detected by a sensor constituted by a light emitting portion 61 and a light receiving portion 62. In this way, an adjustment is made so that a distance between the front end of the top sheet S1 and a sheet attracting and conveying belt allows the sheet attracting and conveying belt to reliably attract the sheet S1.
In such a conventional sheet feeding apparatus blowing air to the side face of the sheet stack, in the case of a sheet having a high attracting force and a large thickness, strong air, in other words, a large quantity of air (high-speed air) should be blown for reliably raising the sheet to separate it because the sheet is heavy especially under a high-humidity environment.
Even the same sheet may have a low attracting force depending on storage conditions, and in the case of a sheet having a low attracting force and a small thickness, the sheet can sufficiently be raised with weak air, in other words, a small quantity of air especially under a low-humidity environment. In the case of such a sheet, the sheet falls into disorder in a sheet stacking portion if the quantity of air is too large.
If the sheet thus falls into disorder, so called skew conveying in which the sheet is conveyed on the skew, so called a lateral registration shift in which the sheet is shifted in a direction perpendicular to the conveyance direction, or the like occurs, and the sheet cannot appropriately be conveyed. If the sheet cannot appropriately be conveyed as described above, an image cannot appropriately be formed on the sheet.